Westminster Project 1.4 - 1.5

4. The authority of the Holy Scripture, for which it ought to be believed and obeyed, does not depend on the testimony of any human or of the Church, but wholly on God (who is truth itself), the author of it. Therefore it is to be received because it is the Word of God. 

5. We may be moved and induced to a high and reverent esteem of the Holy Scripture by the testimony of the Church; other arguments by which it abundantly evidences itself to be the Word of God are: 

the heavenliness of the subject matter, 
the effects of its teaching, 
the majesty of its style, 
the agreement of all its parts, 
the scope of the whole (which is to give all glory to God), 
the full presentation it makes of the only way of the salvation of mankind, 
and its many other incomparable excellences, and the entire perfection of it. 

Nevertheless, our full persuasion and assurance of the unerring truth and divine authority of Holy Scripture is from the inward work of the Holy Spirit, bearing witness by and with the Word in our hearts. 
(WCF 1.4-5)

How do we know that the 66 books of the Protestant canon are the inspired Word of God? How can we be sure that what we have is truly from God? This is precisely what the confession sets out to answer for us here.
We know that the Bible is from God because God Himself has testified to this (1 John 5:9). Why is that important? Because if the authority of Scripture rested in the Church, a pastor, a pope, or a council, then the final authority would lie in human hands—not in God. That’s the key argument being made.
In other words, we have the Bible because God Himself has spoken (2 Timothy 3:16–17; cf. 1 Peter 1:10–11, 2 Peter 1:20–21). The Church’s role was not to create Scripture, but simply to receive, collect, and recognise God’s revelation for what it is (Romans 3:2; 1 Thessalonians 2:13; 2 Peter 3:15b–16). That is why we dare not add to or take away from it (Deuteronomy 4:2; Revelation 22:18–19).
In light of this, it follows that the Church’s authority on any matter does not come from itself, but from God alone.
This has major implications. We believe and trust the Bible not because someone told us to, but because God Himself has spoken. We dare not ignore our Creator (Jeremiah 7:22–24; Hosea 4:6; Romans 1:18). This truth holds everyone—whether church, denomination, teacher, or preacher—accountable. It reminds us that God’s Word stands above all human authority. It judges their work and our lives—not the other way around.
However, some may not be fully convinced by that argument. That’s why the confession encourages us to look for “the fingerprints of God” in Scripture for ourselves. Consider:
No other body of work so consistently reveals who God is.
Its teachings are not shaped by human opinion.
There are no true contradictions.
It lifts the reader’s mind and heart heavenward.
The plan of salvation is both clear and entirely unlike any human scheme.
None of this is a mere coincidence. The Bible is a library of 66 books, written over 1,500 years by around 40 authors, in three languages, across many cultural contexts—yet it speaks with one voice about one God, one covenant, one Saviour, one way, and one truth.
The confession (1.5) ends with this thought: The Church may lead a horse to water—and even salt the oats to make it thirsty—but ultimately, it is the Holy Spirit of God who enables someone to drink deeply and recognise Scripture for what it truly is: God’s very Word to us.